The rapid development of computers has resulted in an increased information industry, and a substantial amount of research has been devoted to developing methods for improving the output of large amounts of recorded data. Silver halide photographic materials which are compatible with reversal processing are used as recording materials in this field. The processing step in the reversal development method involves the formation of a negative image by means of a first development process. The negative image is then subjected to a bleaching process without a fixing process, and the reduced silver of the negative image is removed. The residual undeveloped silver halide is then exposed and a positive image is formed by performing a second development. However, the processing step of this method is complicated and the rate at which the finished film is obtained is low. Additionally fluctuations are likely to arise in the maximum density (D.sub.max) and the minimum density (D.sub.min) of the film. Moreover, a powerful bleaching agent, such as potassium dichromate, must be used in the bleaching bath, causing pollution problems.
The photographic method by which direct positive images are obtained without the need for a reversal processing step or a negative film is well known as a means of avoiding problems of this type.
The methods used to form positive images with conventional direct positive silver halide photographic materials divide essentially into two main types, omitting special cases.
The first type of method involves the use of a pre-fogged silver halide emulsion in which the fog nuclei (latent image) in the exposed parts are destroyed by means of solarization or a Herschel effect, for example, and a direct positive image is obtained after development.
In the other type of method, an internal latent image type silver halide emulsion which has not been pre-fogged is used. After imagewise exposure, these materials are subjected to surface development either after a fogging step or during a fogging step, and a direct positive image is obtained.
The photosensitive nuclei of internal latent image type silver halide photographic emulsions are principally within the silver halide grains. Further, the latent image, on exposure, is formed principally within the grains.
The latter method generally has a higher photographic speed than the former method and therefore can be used in applications in which high speeds are required. The present invention is concerned with methods of this later type.
Various techniques are already known in this field of technology. For example, disclosures have been described principally in the specifications of U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322 (2,497,875), 3,761,266, 3,761,276 and 3,796,577, and British Patents 1,151,363 and 1,150,553 (1,011,062). Comparatively fast photographic photosensitive materials of the direct positive type can be made using these known methods.
Furthermore, details of the mechanism by which direct positive images are formed is disclosed, for example, in T.H. James, The Theory of the Photographic Process, Fourth Edition, Chapter 7, pages 182 to 193, and in U.S. Pat. No. 3,761,276.
Thus, fog nuclei are produced selectively only on the surface of the silver halide grains in the unexposed areas because of the surface desensitizing action which results from the internal latent image which has been formed within the silver halide grains as a result of the initial imagewise exposure. A photographic image (a direct positive image) can usually be formed in the unexposed parts by a surface development process.
Known means of forming fog nuclei selectively as mentioned above include the so-called light fogging methods in which the whole surface of the photosensitive layer is subjected to a second exposure (for example, British Patent 1,151,363) and chemical fogging methods in which nucleating agents are used. The latter methods have been disclosed, for example, in Research Disclosure, Vol. 151, No. 15162 (published November 1976), pages 72 to 87.
The conventional chemical fogging methods are such that high pH values of 12 or above are used initially to realize the effect of the nucleating agent, and so deterioration of the developing agent due to aerial oxidation is likely to occur under these high pH conditions. There is consequently the disadvantage of a pronounced loss of development activity. There is a further disadvantage in that the rate of development is slow and the processing time is prolonged, and an especially long processing time is required, when low pH development baths are used.
On the other hand, in the case of the light fogging method, there is no need for high pH conditions, which is comparatively useful in practice. However, there are various technical problems because of the wide purpose of photographic fields in which these materials are used. That is to say, the light fogging method is based on the formation of fog nuclei resulting from the photodegradation of silver halides and so there are differences in the appropriate exposure intensity and exposure level, depending on the type of silver halide which is being used and its characteristics. Consequently, it is difficult to achieve a constant level of performance, and there is the further disadvantage that the developing apparatus is complicated and expensive. There is also the disadvantage that the processing time is prolonged.
Thus, it is difficult to obtain good direct positive images in a stable manner using the conventional fogging methods. Compounds which exhibit a nucleating action at pH values of less then 12 have been suggested in JP-A-52-69613 and U.S. Pat. Nos. 3,615,615 and 3,850,638 as a means of resolving these problems, but these nucleating agents act upon the silver halide during storage of the sensitive materials prior to processing. Further, they are themselves degraded, and so there is the disadvantage that the maximum image density after processing is in fact reduced. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".)
The use of hydroquinone derivatives to increase the development rate of medium densities has been disclosed in U.S. Pat. No. 3,227,552. However, the rate of development is still inadequate even when these derivatives are used and, in particular, only inadequate rates of development can be attained with development at pH of less than 12.
Furthermore, the addition of mercapto compounds which contain carboxylic acid groups or sulfonic acid groups to increase the maximum image density has been disclosed in JP-A-60-170843. However, the effect of adding these compounds is slight.
Reduction of the minimum image density and prevention of the formation of re-reversal negative images by processing in processing baths (pH 12.0) which contain tetrazaindene based compounds in the presence of nucleating agents has been described in JP-A-55-134848. However, in this method neither the maximum image density nor the rate of development is high.
Furthermore, the addition of triazolinthione or tetrazolinthione based compounds as anti-foggants to sensitive materials with which direct positive images are formed using a light fogging method has been disclosed in JP-B-45-12709. (The term "JP-B" as used herein signifies a "examined Japanese patent publication".) However, it is not possible even with this method to achieve high maximum image densities and rapid development rates.
Thus, as yet there is no technique for obtaining in a short period of time a direct positive image which has a high maximum image density and a low minimum image density.
Furthermore, there is the general problem that re-reversal negative images are often formed in high speed direct positive emulsions with high exposure levels (i.e., high exposure intensity). The prevention of re-reversal negative image formation is especially important with the high intensity exposures which arise from short exposures and the high speeds required for COM purpose films.
Techniques for overcoming the aforementioned problems have been disclosed in JP-A-60-73625, JP-A-60-443, JP-A-63-8704 and JP-A-63-106656, but these methods can not prevent the formation of re reversal negative images.